BALANCE SHEET COMPONENTS - Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Jun. 29, 2018	Sep. 29, 2017
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	$ 155.8
Amortized cost	10.9	$ 60.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	10.9	60.1
Original CPTC loans
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		47.4
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		47.4
DRTC Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		8.3
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		8.3
APTC Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6.3
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Fair Value	6.3
GPTC Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	4.6	4.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	4.6	$ 4.4
Money Market Funds
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	$ 155.8

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 25
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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